Fuel and driveline efficiency is a controlling factor in the design of vehicle components. Specifically, designers of all-wheel drive and four-wheel drive vehicles weigh fuel efficiency for daily driving against performance in extreme roadway or off-highway conditions. Motor vehicles are driven by a transmission that transmits rotational torque to a power transfer unit (also known as a power take-off unit) through a torque transmitting shaft. The power transfer unit ultimately drives a plurality of axles that can be divided into those with a hang-on four-wheel drive, wherein a primary axle is driven permanently and a secondary axle is connected if required, and those with a permanent four-wheel drive, wherein both axles are driven permanently. The design of the driveline is largely influenced by the arrangement of the engine in the motor vehicle, i.e. whether it is arranged in the front or at the rear and whether it is positioned in the longitudinal or transverse direction. At the same time, stringent packaging requirements exist regarding size, weight, and assembly costs of such systems.
Power transfer units are commonly utilized in front-wheel drive based all-wheel drive systems. A power transfer unit transmits the torque from the transmission to a propshaft, which in turn delivers power to the rear wheels. Most power transfer units are always in a ready state, commonly controlled by a slipping clutch near the rear axle, and yet are utilized only a small fraction of the time during driving. However, in this “ready state”, the existing power transfer units exhibit a full time drain to fuel efficiency with only a part-time benefit
In typical four-wheel drive based layouts, the engine is longitudinally mounted, and the power transfer unit locks to simultaneously drive both a front and rear shaft, which in turn activate the rotation of front and rear external differentials to drive a shaft extending to each wheel. The typical external differential is a single 90° gear set that drives the wheels. Alternatively, in a typical front-wheel drive based all-wheel drive hang on layout, the engine is transversely mounted, and the front differential is included within or as a direct part of the transmission assembly. The vehicle can be driven in an all-wheel drive configuration by transferring power from the power transfer unit and delivering it to the rear wheels through a single gear set in a differential unit while driving the front wheels. This can be accomplished by several known couplings, including on-demand couplings.
In vehicles where it is desirable to provide a very low range (creep) drive mode, however, the vehicle transmission may not provide a low enough drive ratio to enable a very low range drive ratio. This may be particularly true in cases of conventional transmission designs where the differential is located internally to the transmission with no other gear reduction mechanism provided.
Thus, there exists a need for efficiently transitioning between a two-wheel drive system to a four-wheel drive or all-wheel drive system by transferring rotational torque and reducing the drive speed of the power transfer unit to enable a low range drive ratio, when engaging all four wheels, while maintaining a tight component package footprint.